1. Field of the Invention
The present invention relates to an integrated device and fabricating method thereof. More particularly, the present invention relates to an integrated device and its fabricating method for increasing a tuning range using a film bulk acoustic resonator (FBAR) and a tunable capacitor integrated together and reducing a parasitic resistance occurring from a discrete connection.
2. Description of the Related Art
Recently, as mobile communication devices, such as cellular phones, are prevalently used, effort has been made to enhance the performance of mobile communication devices and fabricate them to have smaller size and lighter weight. In response, research is conducted to improve the performance of components of the mobile communication device and miniaturize the components with light weight at the same time.
One of the crucial components of the mobile communication device is a duplexer. The duplexer, which is one of representative devices using a filter in a composite manner, adequately separates signals transmitted and received over a single antenna in a frequency division duplex (FDD) communication system, thus enabling the device to share the antenna efficiently.
A basic structure of the duplexer includes a transmit filter and a receive filter besides an antenna. The transmit filter is a band pass filter which passes only signals in a frequency band to be transmitted, and the receive filter is a band pass filter which passes only signals in a frequency band to be received. The duplexer differently regulates frequencies passed through the transmit filter and the receive filter and thus allows transmission and reception on the single antenna.
The transmit filter and the receive filter, forming the basic structure of the duplexer, can employ a film bulk acoustic resonator (FBAR). It is known that the FBAR, which can be implemented with small size and light weight, is the dominant means for configuring a filter suitable for high power. The FBAR can be manufactured with minimum cost in miniature size. Also, the FBAR can realize high Quality Factor (Q) value which is an important property of the filter and can be used in a micro-frequency band. In particular, the FBAR can even implement personal communication system (PCS) band and digital cellular system (DCS) band.
The FBAR is fabricated by depositing a lower electrode, a piezoelectric layer, and an upper electrode in that order, to generate resonance when an external electric field is applied. Specifically, when the time-variant electric field is induced by applying electric energy to the upper and lower electrodes of the FBAR, the piezoelectric layer generates the piezoelectric effect which transforms the electric energy to an acoustic mechanical energy to thus generate the resonance. Since the FBAR passes only signals in a specific band based on the resonant frequency, it functions as a band pass filter.
In the mean time, as communication devices become miniaturized and complicated, there has been a demand for small terminals capable of using multiple frequency bands. To use multiple frequency bands at one terminal, a filter bank using multiple filters can be employed. However, the filter bank hinders the recent trend toward size reduction. To avoid this problem, a tunable filter is adopted. The filter bank can be replaced by one or two tunable filters of which the frequency changes by about 30% because of the voltage. A microelectromechanical systems (MEMS) resonator capable of tuning enables the implementation of a tunable filter. It is a FBAR using bulk resonance characteristics that shows good characteristics in the 2˜5 GHz band with the smallest size up to now. Also, since the FBAR is low-priced and uses a silicon substrate having integration compatibility with an IC, it is easy to integrate with a MEMS tunable L/C for frequency tuning.
FIGS. 1A and 1B depict a conventional tunable FBAR.
FIG. 1A shows a structure 10 in which a tunable device and a FBAR are linked to each other. Since the FBAR 11 and the tunable capacitor 12 are not integrated but discretely connected, parasitic resistance may occur and there is a limit to the miniaturization.
FIG. 1B shows a structure 20 in which a tunable capacitor and a FBAR are integrated together. The capacitance is varied by vertically moving the FBAR 22 which is fixed in the cantilever structure. In this situation, disadvantageously, the tuning rate is limited to 1:1.5 due to the pull-in effect and the FBAR characteristics (Q value) are subject to variation because the FBAR 22 moves as a whole.